Potassium Phosphate Recovery from Incinerated Ash of Sewage Sludge

Journal of Materials Science and Engineering B 7 (7-8) (2017) 149-154 doi: 10.17265/2161-6221/2017.7-8.003 D DAVID PUBLISHING Potassium Phosphate Recovery from Incinerated Ash of Sewage Sludge Masaaki Takahashi 1, Yukimasa Takemoto 1, Eiji Yuki 2 and Koichi Seno 2 1. Faculty of Environmental and Information Science, Yokkaichi University, Yokkaichi 512-8512, Japan 2. Mie Chuo Kaihatsu Co., Ltd, Hachiya 4713, Iga Mie 518-1152, Japan Abstract: In order to establish a phosphorus recovery method, acidic treatment is considered useful for its high extraction rate, however, phosphorus recovered by acidic treatment is mainly composed of aluminum phosphate. In order to remove the aluminum component from aluminum phosphate, alum formation processes were investigated. The phosphorus component which was extracted from ash, was mixed with potassium sulfate in combination with sulfuric acid, and formed alum (aluminum potassium sulfate), which was separated by crystallization. The aluminum removed phosphorus was recovered by addition of potassium hydroxide. The recovered phosphorus was considered to be a mixture of potassium phosphate and potassium sulfate by X-ray analysis. Key words: Incinerated ash, sewage sludge, acid treatment, potassium phosphate, alum. 1. Introduction Sewage sludge contains significant amounts of phosphorus, and in order to recover the phosphorus from it, many investigations have been carried out [1-5]. Phosphorus in the sludge is considered to be existing mainly in the form of aluminum phosphate [6], which has few uses, and the existence of aluminum components make a bad influence on many plants, therefore, some recovery methods which can recover phosphorus with low aluminum content are recommended. In acidic treatments, almost all of the phosphorus in the ash can be extracted from the ash, however, the extracted phosphorus consists mainly of aluminum phosphate [7]. Aluminum ions react with monovalent cations like NH + 4, K +, Na + under existence of H 2 SO 4, and form alum, as is widely known. Some alum like AlNH 4 (SO 4 ) 2 or AlK(SO 4 ) 2 have low solubility in water at low temperature. Aluminum phosphate reacts with these cations and forms alum which can be separated by crystallization resulting in the formation of H 3 PO 4 [8, 9] as shown in Eq. (1). We investigated a Corresponding author: Masaaki Takahashi, researcher, research field: environmental technology. phosphorus recovery method using potassium sulfate instead of ammonium sulfate [10]. 2AlPO 4 + 3H 2 SO 4 + K 2 SO 4 = 2AK(SO 4 ) 2 + 2H 3 PO 4 (1) 2. Method 2.1 Ash The incinerated ash that was discharged from Yokkaichi municipal waste water treatment facility was used in the experiment. The ash is made from sewage sludge which is incinerated in a fluidized bed incinerator at 850 C. The chemical components of the ash are shown in Table 1. 2.2 Phosphorus Recovery Method The outline of the phosphorus recovery method is shown in Fig. 1. Phosphorus in the ash was extracted using an aqueous solution of H 2 SO 4. Powder of potassium sulfate was added to the extract at room temperature, and the mixture was cooled to a low temperature (under 5 C). After 1 day, crystal of alum is formed, and separated by filtration. The formed phosphoric acid is expected to contain significant amounts of non-reacted aluminum phosphate and H 2 SO 4,

150 Potassium Phosphate Recovery from Incinerated Ash of Sewage Sludge Table 1 Chemical composition of the ash. Component Composition (%) Component Composition (%) Al 2 O 3 12.59 K 2 O 2.64 SiO 2 36.78 CaO 9.42 P 2 O 5 17.21 Fe 2 O 3 12.05 SO 3 1.21 Others 8.0 H 2 SO 4 +Water K 2 SO 4 KOH Filtrate Ash Extraction Extract Mixing Crystallization Temp. 5 C ph 4~5 Vaporization H 2 SO 4 Filtration Treated ash Crystal Residue Phosphorus (KAl(SO 4 ) 2 ) (Non-reacted AlPO 4 ) (KH 2 PO 4 ) Fig. 1 Recovery method. and separation of these contaminants in acidic condition is thought to be difficult. In order to confirm the reaction, potassium hydroxide was added to the extract to adjust the ph 4~5 which is considered to be the best condition for the separation of non-reacted phosphorus (AlPO 4 or FePO 4 ) [11]. Non-reacted phosphorus was precipitated, and removed by filtration as residue, and the aluminum removed phosphorus formed a sulfate compound, and can be recovered by vaporization of the filtrate. 2.3 Preparation of the Acidic Phosphorus Containing Water The phosphorus containing water was prepared by acidic extraction of the ash. In consideration of the chemical composition, the amount of H 2 SO 4 in the acidic phosphorus containing water was considered to be small to make a crystallization of alum, and appropriate amounts of H 2 SO 4 was added to the extract to adjust the condition. In order to make a crystallization, about 100 g/l of the aluminum phosphate is considered necessarily in consideration of the solubility of aluminum potassium sulfate (AlK(SO 4 ) 2 12H 2 O) [12]. However, usually applied extraction rate (solid-liquid ratio 1:10) is expected result in a low concentration of aluminum phosphate in the extract. Therefore, to solve the matter, we adopted a high solid-liquid ratio and counter current extraction method [13], and possible aluminum phosphate concentration (about 80 g AlPO 4 /L) can be attained by adoption of solid-liquid ratio 1:3. 3. Result and Discussion 3.1 Phosphorus Extract Rate Raw ash and phosphorus extracted ash were analyzed using X-ray analyzer followed by dehydration. Phosphorus and aluminum in the ash was degreased by acid treatment, and the SO 3 component was increased depending on the reaction of H 2 SO 4 and calcium component in the ash. The phosphorus extraction rate was calculated from the elemental composition (Fig. 2), and was considered about 80%. 3.2 Addition Rate of the Potassium Sulfate In order to find an optimal addition rate of potassium sulfate, 100 ml of aluminum phosphate solution (P: 3.0%, H 2 SO 4 : 10%) extracted from ash as mentioned before, was mixed with sulfuric acid

Potassium Phosphate Recovery from Incinerated Ash of Sewage Sludge 151 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% others Fe2O3 CaO K2O SO3 P2O5 SiO2 Al2O3 0% Raw ash TreatedAsh K Alum Residue Phosphorus Fig. 2 Chemical composition of the recovered materials. (9 g), and 6 g to 15 g of potassium sulfate powder was added, and dissolved at room temperature. Later, the mixture was cooled to 5 C, 1 day later, crystals formed (which are expected to be alum) were separated by filtration using filter paper (ADVANTEC Toyo Roshi Kaisya LTD). The amount of the formed crystal was increased by the addition of potassium sulfate toward the equivalent ratio. When the addition rate reaches chemical equivalent amount, the increasing rate of the formed crystal tends to increase linearly as shown in Fig. 3 which is considered to be the low solubility of potassium sulfate in water at 5 C. From the result, the most suitable addition rate is considered to be 7 g more than the equivalent amount (surplus). 3.3 Addition Rate of Sulfuric Acid In order to find an optimal addition rate of sulfuric acid, 100 ml of aluminum phosphate solution (P: 1.6%, H 2 SO 4 : 9.1%) extracted from ash was mixed with potassium sulfate (10 g), and 3.6 g to 7.3 g of sulfuric acid was added at room temperature. Later, the mixture was cooled at 5 C, 1 day later, crystals formed and were separated as the same way as mentioned before. The amount of the formed crystal was also increased by the addition of sulfuric acid toward the equivalent ratio. When the addition rate reaches 2 g larger than the chemical equivalent amount, the increasing rate of the crystal tends to plateau (Fig. 4). The most suitable addition rate is considered to be 2 g more than equivalent amount (surplus). 3.4 Composition of the Recovered Phosphorus In order to confirm the formation of the potassium phosphate, 300 ml of phosphorus containing water extracted from ash using H 2 SO 4 was prepared. The phosphorus and H 2 SO 4 concentration of the phosphorus containing extract is estimated to be P: 1.5%, H 2 SO 4 : 3.6%. Later, 16 g of H 2 SO 4 and 30 g of K 2 SO 4 were added to the phosphorus containing extract, and dissolved at room temperature. The mixture was cooled for 24 hours at 5 C, and formed crystal (38 g) was separated using filter paper. The crystal formed by cooling was expected to be alum, and dehydrated at 180 C for analysis. In order to recover phosphorus, 28 g of potassium hydroxide was added to the filtrate, and adjusted at the

152 Potassium Phosphate Recovery from Incinerated Ash of Sewage Sludge (g) 40 (g) 25 30 20 20 10 0 Equivalent ratio 0 10 20 30 15 10 5 0 Equivalent ratio 0 5 10 15 K 2 SO 4 addition rate(g) H 2 SO 4 addition rate (g) Fig. 3 Relation with formed crystal and K 2 SO 4. ph 4~5. Brown precipitate was formed which was expected as non-reacted phosphate, and removed by filtration using filter paper as a residue, and dried at 105 C (amount: 17 g). The phosphorus which was expected to be potassium phosphate was recovered by evaporation of the filtrate (56 g). These recovered materials were analyzed using XRD (X-ray diffraction) and X-ray analyzer (shown in Fig. 2). The recovered crystal was confirmed to be KAl(SO 4 ) 2 12H 2 O (shown in Fig. 5). A small amount of P 2 O 5 found in Fig. 4 Relation with formed crystal and H 2 SO 4. the crystal is considered to be derived from formed phosphoric acid, and remained in the crystal. The residue was mainly composed of P 2 O 5, SO 3, Fe 2 O 3, and K 2 O, and most of the aluminum component was removed. The residue is considered to consist of ferric phosphate and potassium sulfate which remained in the residue from chemical composition. Aluminum component was mostly removed by crystallization, however, significant amounts of phosphorus remained in the residue. Counts KAl(SO 4 )12H 2 O 2θ Fig. 5 XRD of the recovered crystal.

Potassium Phosphate Recovery from Incinerated Ash of Sewage Sludge 153 Counts K 2 SO 4 K 3 Na(SO 4 ) 2 K 2 HPO 4 Fig. 6 XRD of the recovered phosphorus. 2θ The ash contains significant amounts of iron [14], and the iron component in the extract forms ferric potassium sulfite (FeK(SO 4 ) 2 ) which is soluble in water, and is considered to remain in the residue. The recovered phosphorus is mainly composed of K 2 O, P 2 O 5 and SO 3. The recovered phosphorus is considered to be a mixture of potassium phosphate (KH 2 PO 4 ) and potassium sulfate (Fig. 6). 4. Conclusions In order to recover phosphorus from the ash of sewage sludge with low aluminum content, a recovery method by alum formation was investigated. The aluminum component which was extracted with phosphorus from ash, formed alum by the addition of K 2 SO 4, and was separated from phosphorus, and recovered as potassium phosphate. Alum can be used in food additives and also many industrial raw materials. The recovered potassium phosphate contains potassium sulfate. Potassium phosphate and potassium sulfate can be used as a fertilizer [15], therefore, this mixture is expected to be useful for agriculture. However, significant amounts of phosphorus cannot be recovered, and remains in the residue, and further study will be needed to attain a higher phosphorus recovery rate. References [1] Shima, H., and Takahashi, M. 1997. Gekkan Water. Gekkan [Mizu] Publishing, 39-7 (552): 36-40. (In Japanese). [2] Okano, K., Umemoto, M., Kagami, J., Honda, K., Ohtake, H., Miura, K., Akemoto, T., and Toda, M. 2013. Novel Technique for Phosphorus Recovery from Aqueous Solutions Using Amorphous Calcium Silicate Hydrates (A-Cshs). Water Research 47 (7): 2251-9. [3] Keisuke. Nakahara: Verification Process for Recovery of Phosphorus/Phosphoric Acid from Sewage Sludge

154 Potassium Phosphate Recovery from Incinerated Ash of Sewage Sludge Ash. Industrial water 514: 2-10. [4] Keisuke. 2003. Nakahara: Verification Process for Recovery of Phosphorus from Sewage Sludge. Resources Prosing 50 (2): 68-73. [5] Sonoda, K., Tokukura, K., Mori, T., Komazawa, I., and Wada, T. 1996. Phosphoric Acid Recovering from Incinerated Ashes of Sewage Treatment Sludge. In Proceedings of the 7th Annual Conference of the Japan Society of Waste Management Expert 6-9: 267-9. [6] Ingham, J., Ryan, J., Keyakida, E., and Ri, J. 1996. Phosphorus Metal Recovery from Sewage Treatment Sludge. In Proceedings of the 7th Annual Conference of the Japan Society of Waste Management Expert, 280-2. [7] Masaaki Takahashi, Yukimasa Takemoto, and Ken Oonishi. 2013. Recovery of Aluminum Phosphate Using Magnesium Hydroxide from Incinerated Ash of Sewage Sludge. Journal of Material Science and Engineering A 3 (10): 710-3. [8] Takahashi, M., Kato, S., Shima, H., Sarai, E., Ichioka, T., Hayakawa, S., and Miyajiri, H. 2004. Technique for Recovering Phosphorus Salt from Incinerated Ash of Sewage Treatment Sludge. Transactions of Materials Research Society of Japan 29 (5): 2149-52. [9] Takahashi, M., Takemoto, Y., and Onishi, K. 2014. Ammonium Phosphate Recovery from Incinerated Ash of Sewage Sludge. In Proceedings of the 25th Annual Conference of the Japan Society of Waste Management Expert, 231-2. [10] Takahashi, M., Kato, S., Onari, Y., and Enjyoji, H. 2002. Study on Technology to Recover Various Element from Incinerated Ash (5) Technology for Recovering Phosphorus Salt from Incinerated Ash of Sewage Treatment Sludge. Annual Report of Mie Prefecture Health and Environment Research Institute 4: 27-31. [11] Takahashi, M., Kato, S., Shima, H., Sarai, E., Ichioka, T., Hayakawa, S., and Miyajiri, H. 2001. Technology for Recovering Phosphorus from Incinerated Wastewater Treatment Sludge. Chemosphere 44: 23-9. [12] Taimei Chemical Co. Ltd. Product Info. http.//mmm.taimei-chem.co.jp/05.html. Accessed Jun. 1st 2017. [13] Takahashi, M., Takemoto, Y., and Onishi, K. 2013. Phosphorus Recovery from Charcoal of Sewage Sludge Using NaOH. International Journal for Sustainable Innovations 3 (1): 37-44. [14] Nishio, T. 2011. Applicability and Efficiency of Heat Treatment for Phosphorus Recovery from Sewage Sludge in Osaka City. Annual Report of Osaka City Institute of Public Health and Environmental Science 74: 43-8. [15] Taimei Chemicals Co. Ltd, http://www.taimei-chem.co.jp/product/06.html. Accessed Jun. 1st 2017.